Host device with active-active storage aware path selection

ABSTRACT

An apparatus comprises at least one processing device that includes a processor coupled to a memory. The processing device is configured to control delivery of input-output (TO) operations from a host device to at least first and second storage systems over selected ones of a plurality of paths through a network, the first and second storage systems being arranged in an active-active configuration relative to one another. The processing device is further configured to identify one or more logical storage devices that are each accessible via at least first and second different ones of the paths to respective ones of the first and second storage systems, and to modify path selection for IO operations directed to the one or more identified logical storage devices relative to path selection for IO operations directed to one or more other logical storage devices. The processing device illustratively comprises at least a portion of the host device.

FIELD

The field relates generally to information processing systems, and moreparticularly to storage in information processing systems.

BACKGROUND

Storage arrays and other types of storage systems are often shared bymultiple host devices over a network. Applications running on the hostdevices each include one or more processes that perform the applicationfunctionality. The processes issue input-output (IO) operations fordelivery over paths from the host devices to storage ports of thestorage system. The storage ports are typically limited in number andeach has limited resources for handling TO operations received from thehost devices. Different ones of the host devices can run differentapplications with varying workloads and associated TO patterns. Suchhost devices also generate additional TO operations in performingvarious data services such as replication and migration so as to meetbusiness continuity requirements. Conventional host device multi-pathingarrangements are in some situations unable to deal adequately with theseand other variabilities in TO processing behavior. For example, someexisting multi-path layers implement a static path selection approachthat does not produce optimal results in all situations, includingsituations in which logical storage volumes or other logical storagedevices are replicated across multiple storage arrays arranged in anactive-active configuration.

SUMMARY

Illustrative embodiments provide active-active storage aware pathselection in a host device. The path selection is illustratively“active-active storage aware” in that it can determine whether or not agiven logical storage volume or other logical storage device isreplicated across multiple storage arrays in an active-activeconfiguration, and adjust the selection process accordingly so as toimprove performance. The paths illustratively comprise paths through astorage area network (SAN) or other type of network over which the hostdevice communicates with multiple storage arrays or other types ofstorage systems arranged in an active-active configuration.

In some embodiments, the active-active storage aware path selection isimplemented in a multi-path layer that comprises at least one multi-pathinput-output (MPIO) driver configured to process IO operations of atleast one host device that communicates with multiple storage arrays orother types of storage systems.

The multi-path layer in such arrangements can be configured, forexample, to detect particular logical storage volumes or other logicalstorage devices that are replicated across multiple storage arrays orother storage systems in an active-active configuration, such that themulti-path layer thereby becomes “aware” of the active-activeconfiguration for those storage devices, and to modify the manner inwhich it performs path selection so as to provide enhanced loadbalancing between the multiple storage arrays under such conditions.

For example, the active-active storage aware path selection can beadvantageously configured in some embodiments to ensure that pathselection in the host device does not adversely impact data prefetchingdecisions, IO pattern recognition, automated storage tiering, machinelearning or other similar IO-based functionality in the individualstorage arrays. As a result, improved performance in processing of IOoperations by the storage arrays is achieved.

In one embodiment, an apparatus comprises at least one processing devicethat includes a processor coupled to a memory. The processing device isconfigured to control delivery of IO operations from a host device to atleast first and second storage systems over selected ones of a pluralityof paths through a network, with the first and second storage systemsillustratively being arranged in an active-active configuration relativeto one another. The processing device is further configured to identifyone or more logical storage devices that are each accessible via atleast first and second different ones of the paths to respective ones ofthe first and second storage systems, and to modify path selection forIO operations directed to the one or more identified logical storagedevices relative to path selection for IO operations directed to one ormore other logical storage devices.

The processing device illustratively comprises at least a portion of thehost device, and in some embodiments includes an MPIO driver thatperforms at least a portion of the identification and modification.

The paths are illustratively associated with respective initiator-targetpairs, with the initiators being implemented on the host device and thetargets being implemented on the first and second storage systems. Theinitiators of the initiator-target pairs illustratively compriserespective host bus adaptors (HBAs) of the host device and the targetsof the initiator-target pairs illustratively comprise respective storagearray ports of the first and second storage systems. Other types ofinitiators and targets can be used in other embodiments.

In some embodiments, identifying one or more logical storage devicesthat are each accessible via at least first and second different ones ofthe paths to respective ones of the first and second storage systemscomprises sending commands on respective ones of the paths over which agiven logical storage device is accessible to the host device, obtainingfrom at least one of the first and second storage systems informationregarding the given logical storage device responsive to the commands,and determining whether or not the given logical storage device isaccessible via at least first and second different ones of the paths torespective ones of the first and second storage systems based at leastin part on the obtained information.

The commands in such an embodiment illustratively comprise respectivecommands of a storage protocol that the host device utilizes tocommunicate with the first and second storage systems. For example, thecommands may comprise respective inquiry page commands each of whichwhen received by one of the first and second storage systems causes adesignated page comprising logical storage device constituencyinformation to be returned by that storage system to the host device.

The obtained information for one of the commands sent on one of thepaths in some embodiments comprises an identifier of the given logicalstorage device and an identifier of its corresponding one of the firstand second storage systems.

In arrangements of this type, the given logical storage device may beidentified as one of the one or more logical storage devices that areeach accessible via at least first and second different ones of thepaths to respective ones of the first and second storage systemsresponsive to the obtained information for one of the commandscomprising an identifier of the first storage system and the obtainedinformation for another one of the commands comprising an identifier ofthe second storage system.

In some embodiments, modifying path selection for IO operations directedto the one or more identified logical storage devices relative to pathselection for IO operations directed to one or more other logicalstorage devices comprises separating a given one of the one or moreidentified logical storage devices into at least first and secondportions, sending IO operations directed to the first portion over oneor more selected paths to the first storage system, and sending IOoperations directed to the second portion over one or more selectedpaths to the second storage system.

Path selection for the one or more other logical storage devices is notconfigured in this manner but instead, for example, treats each suchother logical storage device as a unitary device rather than as aseparated device in selecting paths for delivery of IO operationsdirected to that other logical storage device.

The above-noted separating of a given identified logical storage deviceinto at least first and second portions illustratively comprisesseparating the given identified logical storage device into n contiguousportions each representing a corresponding fraction 1/n of a contiguouslogical address space of the identified logical storage device, where nis an even integer greater than or equal to two. A wide variety of otherlogical storage device separation arrangements can be used in otherembodiments, and terms such as “separating” and “separated” as usedherein are therefore intended to be broadly construed.

Additionally or alternatively, modifying path selection for IOoperations directed to the one or more identified logical storagedevices relative to path selection for IO operations directed to one ormore other logical storage devices comprises determining a particularportion of a given one of the one or more identified logical storagedevices that is experiencing a relatively high level of IO activityrelative to one or more other portions of the given identified logicalstorage device, separating the particular portion into at least firstand second sub-portions, sending IO operations directed to the firstsub-portion over one or more selected paths to the first storage system,and sending IO operations directed to the second sub-portion over one ormore selected paths to the second storage system. As indicatedpreviously in the context of other embodiments, path selection for theone or more other logical storage devices is not configured in thismanner but instead, for example, treats each such other logical storagedevice as a unitary device rather than as a separated device.

These and other illustrative embodiments include, without limitation,apparatus, systems, methods and computer program products comprisingprocessor-readable storage media.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an information processing system configuredto implement active-active storage aware path selection utilizing amulti-path layer of a host device in an illustrative embodiment.

FIG. 2 is a flow diagram of a process that implements active-activestorage aware path selection utilizing a multi-path layer of a hostdevice in an illustrative embodiment.

FIG. 3 is a block diagram showing multiple layers of a layered systemarchitecture that includes a multi-path layer with active-active storageaware path selection in an illustrative embodiment.

DETAILED DESCRIPTION

Illustrative embodiments will be described herein with reference toexemplary information processing systems and associated computers,servers, storage devices and other processing devices. It is to beappreciated, however, that these and other embodiments are notrestricted to the particular illustrative system and deviceconfigurations shown. Accordingly, the term “information processingsystem” as used herein is intended to be broadly construed, so as toencompass, for example, processing systems comprising cloud computingand storage systems, as well as other types of processing systemscomprising various combinations of physical and virtual processingresources. An information processing system may therefore comprise, forexample, at least one data center or other cloud-based system thatincludes one or more clouds hosting multiple tenants that share cloudresources. Numerous different types of enterprise computing and storagesystems are also encompassed by the term “information processing system”as that term is broadly used herein.

FIG. 1 shows an information processing system 100 configured inaccordance with an illustrative embodiment. The information processingsystem 100 comprises at least first and second host devices 102-1 and102-2, collectively referred to herein as host devices 102. The hostdevices 102 are coupled to a network 104 that comprises at least firstand second switch fabrics 104A and 104B. The host devices 102communicate over the network 104 via switch fabrics 104A and 104B withat least first and second storage arrays 105-1 and 105-2, collectivelyreferred to herein as storage arrays 105. For example, the network 104illustratively comprises at least one storage area network (SAN) and thefabrics 104A and 104B illustratively comprise respective distinct switchfabrics of a set of multiple switch fabrics interconnecting the hostdevices 102 with the storage arrays 105 over the one or more SANs. Eachof the fabrics 104A and 104B in some embodiments is associated with adifferent SAN.

The system 100 is configured such that the first host device 102-1communicates with the first storage array 105-1 over the first switchfabric 104A and communicates with the second storage array 105-2 overthe second switch fabric 104B. Similarly, the second host device 102-2communicates with the first storage array 105-1 over the first switchfabric 104A and communicates with the second storage array 105-2 overthe second switch fabric 104B. Numerous other interconnectionarrangements are possible.

Also, other types of networks can be used in other embodiments, andreferences to SANs, switch fabrics or other particular networkarrangements herein are for purposes of illustration only, asnon-limiting examples.

Although only two host devices 102, two switch fabrics 104A and 104B andtwo storage arrays 105 are shown in the figure, this is by way ofillustrative example only, and other embodiments can include additionalinstances of such elements. It is also possible that alternativeembodiments may include only a single host device.

The host devices 102 illustratively comprise respective computers,servers or other types of processing devices configured to communicatewith the storage arrays 105 over the network 104. For example, at leasta subset of the host devices 102 may be implemented as respectivevirtual machines of a compute services platform or other type ofprocessing platform. The host devices 102 in such an arrangementillustratively provide compute services such as execution of one or moreapplications on behalf of each of one or more users associated withrespective ones of the host devices 102. The term “user” herein isintended to be broadly construed so as to encompass numerousarrangements of human, hardware, software or firmware entities, as wellas combinations of such entities.

Compute and/or storage services may be provided for users under aPlatform-as-a-Service (PaaS) model, an Infrastructure-as-a-Service(IaaS) model and/or a Function-as-a-Service (FaaS) model, although it isto be appreciated that numerous other cloud infrastructure arrangementscould be used. Also, illustrative embodiments can be implemented outsideof the cloud infrastructure context, as in the case of a stand-alonecomputing and storage system implemented within a given enterprise.

The network 104 may be implemented using multiple networks of differenttypes to interconnect the various components of the informationprocessing system 100. For example, the network 104 may comprise aportion of a global computer network such as the Internet, althoughother types of networks can be part of the network 104, including a widearea network (WAN), a local area network (LAN), a satellite network, atelephone or cable network, a cellular network, a wireless network suchas a WiFi or WiMAX network, or various portions or combinations of theseand other types of networks. The network 104 in some embodimentstherefore comprises combinations of multiple different types of networkseach comprising processing devices configured to communicate usingInternet Protocol (IP) and/or other types of communication protocols.

As a more particular example, some embodiments may utilize one or morehigh-speed local networks in which associated processing devicescommunicate with one another utilizing Peripheral Component Interconnectexpress (PCIe) cards of those devices, and networking protocols such asInfiniBand, Gigabit Ethernet or Fibre Channel. Numerous alternativenetworking arrangements are possible in a given embodiment, as will beappreciated by those skilled in the art.

Although illustratively shown as separate from the network 104 in thefigure, at least portions of the storage arrays 105 may be consideredpart of the network 104 in some embodiments. For example, in embodimentsin which the network 104 comprises at least one SAN, the storage arrays105 may be viewed as part of the one or more SANs.

The storage arrays 105-1 and 105-2 comprise respective sets of storagedevices 106-1 and 106-2, collectively referred to herein as storagedevices 106, coupled to respective storage controllers 108-1 and 108-2,collectively referred to herein as storage controllers 108.

The storage devices 106 of the storage arrays 105 illustrativelycomprise solid state drives (SSDs). Such SSDs in some embodiments areimplemented using non-volatile memory (NVM) devices such as flashmemory. Other types of NVM devices that can be used to implement atleast a portion of the storage devices 106 include non-volatile randomaccess memory (NVRAM), phase-change RAM (PC-RAM), magnetic RAM (MRAM),resistive RAM, spin torque transfer magneto-resistive RAM (STT-MRAM),and Intel Optane™ devices based on 3D XPoint™ memory. These and variouscombinations of multiple different types of storage devices may also beused. For example, hard disk drives (HDDs) can be used in combinationwith or in place of SSDs or other types of NVM devices.

A given storage system as the term is broadly used herein can thereforeinclude a combination of different types of storage devices, as in thecase of a multi-tier storage system comprising, for example, amemory-based fast tier and a disk-based capacity tier. In such anembodiment, each of the fast tier and the capacity tier of themulti-tier storage system comprises a plurality of storage devices withdifferent types of storage devices being used in different ones of thestorage tiers. For example, the fast tier may comprise flash drives, NVMdrives or other types of SSDs while the capacity tier comprises HDDs.The particular storage devices used in a given storage tier may bevaried in other embodiments, and multiple distinct storage device typesmay be used within a single storage tier. The term “storage device” asused herein is intended to be broadly construed, so as to encompass, forexample, SSDs, HDDs, flash drives, NVM drives, hybrid drives or othertypes of storage devices.

In some embodiments, at least one of the storage arrays 105illustratively comprises one or more VNX®, VMAX®, Unity™ or PowerMax™storage arrays, commercially available from Dell EMC of Hopkinton, Mass.

As another example, one or both of the storage arrays 105 may compriserespective clustered storage systems, each including a plurality ofstorage nodes interconnected by one or more networks. An example of aclustered storage system of this type is an XtremIO™ storage array fromDell EMC, illustratively implemented in the form of a scale-outall-flash content addressable storage array.

A given storage system as the term is broadly used herein canadditionally or alternatively comprise, for example, network-attachedstorage (NAS), direct-attached storage (DAS) and distributed DAS.

Other additional or alternative types of storage products that can beused in implementing a given storage system in illustrative embodimentsinclude software-defined storage, cloud storage, object-based storageand scale-out storage. Combinations of multiple ones of these and otherstorage types can also be used in implementing a given storage system inan illustrative embodiment.

As mentioned above, communications between the host devices 102 and thestorage arrays 105 within the system 100 may utilize PCIe connections orother types of connections implemented over one or more networks such asnetwork 104. For example, illustrative embodiments can use interfacessuch as Small Computer System Interface (SCSI), Internet SCSI (iSCSI),Serial Attached SCSI (SAS) and Serial Advanced Technology Attachment(SATA). Numerous other interfaces and associated communication protocolscan be used in other embodiments.

The storage arrays 105 in some embodiments may be implemented as part ofcloud infrastructure in the form of a cloud-based system such as anAmazon Web Services (AWS) system. Other examples of cloud-based systemsthat can be used to provide at least portions of the storage arrays 105and possibly other portions of system 100 include Google Cloud Platform(GCP) and Microsoft Azure.

As is apparent from the foregoing, terms such as “storage array” and“storage system” as used herein are intended to be broadly construed,and a given such storage array or storage system may encompass, forexample, multiple distinct instances of a commercially-available storagearray.

The storage devices 106 of the storage arrays 105 are configured tostore data utilized by one or more applications running on one or moreof the host devices 102. The storage devices 106 on one of the storagearrays 105 are illustratively arranged in one or more storage pools. Thestorage arrays 105 and their corresponding storage devices 106 areexamples of what are more generally referred to herein as “storagesystems.” A given such storage system in the present embodiment may beshared by the host devices 102, and in such arrangements may be referredto as a “shared storage system.”

The storage devices 106 of the storage arrays 105 implement logicalunits (LUNs) configured to store objects for users associated with thehost devices 102. These objects can comprise files, blocks or othertypes of objects. The host devices 102 interact with the storage arrays105 utilizing read and write commands as well as other types of commandsthat are transmitted over the network 104.

Such commands in some embodiments more particularly comprise SCSIcommands, although other types of commands may be used in otherembodiments, including commands that are part of a standard command set,or custom commands such as a “vendor unique command” or VU command thatis not part of a standard command set.

A given IO operation as that term is broadly used herein illustrativelycomprises one or more such commands. References herein to terms such as“input-output” and “IO” should be understood to refer to input and/oroutput. Thus, an IO operation relates to at least one of input andoutput. For example, an IO operation can comprise at least one read IOoperation and/or at least one write IO operation. More particularly, IOoperations may comprise write requests and/or read requests directed toa given one of the storage arrays 105.

Each IO operation is assumed to comprise one or more commands forinstructing at least one of the storage arrays 105 to perform particulartypes of storage-related functions such as reading data from or writingdata to particular logical storage volumes or other logical storagedevices of one or more of the storage arrays 105. Such commands areassumed to have various payload sizes associated therewith, and thepayload associated with a given command is referred to herein as its“command payload.”

A command directed by the host device 102-1 to one of the storage arrays105 is considered an “outstanding” command until such time as itsexecution is completed in the viewpoint of the host device 102-1, atwhich time it is considered a “completed” command. The commandsillustratively comprise respective SCSI commands, although other commandformats can be used in other embodiments. A given such command isillustratively defined by a corresponding command descriptor block (CDB)or similar format construct. The given command can have multiple blocksof payload associated therewith, such as a particular number of 512-byteSCSI blocks or other types of blocks.

Also, the term “storage device” as broadly used herein can encompass,for example, a logical storage device such as a LUN or other logicalstorage volume. A logical storage device can be defined in the storagearrays 105 to include different portions of one or more physical storagedevices. The storage devices 106 may therefore be viewed as comprisingrespective LUNs or other logical storage volumes. Logical storagedevices are also referred to herein as simply “logical devices.”

Each of the host devices 102 illustratively has multiple paths to eachof the storage arrays 105 via the network 104, with at least one of thestorage devices 106 of one of the storage arrays 105 being visible tothat host device on a given one of the paths, although numerous otherarrangements are possible. A given one of the storage devices 106 may beaccessible to a given host device over multiple paths. Different ones ofthe host devices 102 can have different numbers and types of paths tothe storage arrays 105.

Different ones of the storage devices 106 of the storage arrays 105illustratively exhibit different latencies in processing of IOoperations. In some cases, the same storage device may exhibit differentlatencies for different ones of multiple paths over which that storagedevice can be accessed from a given one of the host devices 102.

The host devices 102, network 104 and storage arrays 105 in the FIG. 1embodiment are assumed to be implemented using at least one processingplatform each comprising one or more processing devices each having aprocessor coupled to a memory. Such processing devices canillustratively include particular arrangements of compute, storage andnetwork resources. For example, processing devices in some embodimentsare implemented at least in part utilizing virtual resources such asvirtual machines (VMs) or Linux containers (LXCs), or combinations ofboth as in an arrangement in which Docker containers or other types ofLXCs are configured to run on VMs.

Additional examples of processing platforms utilized to implementstorage systems and possibly one or more associated host devices inillustrative embodiments will be described in more detail below.

The host devices 102 and the storage arrays 105 may be implemented onrespective distinct processing platforms, although numerous otherarrangements are possible. For example, in some embodiments at leastportions of the host devices 102 and the storage arrays 105 areimplemented on the same processing platform. The storage arrays 105 cantherefore be implemented at least in part within at least one processingplatform that implements at least a subset of the host devices 102.

The term “processing platform” as used herein is intended to be broadlyconstrued so as to encompass, by way of illustration and withoutlimitation, multiple sets of processing devices and associated storagesystems that are configured to communicate over one or more networks.For example, distributed implementations of the host devices 102 arepossible, in which certain ones of the host devices 102 reside in onedata center in a first geographic location while other ones of the hostdevices 102 reside in one or more other data centers in one or moreother geographic locations that are potentially remote from the firstgeographic location. Thus, it is possible in some implementations of thesystem 100 for different ones of the host devices 102 to reside indifferent data centers than the storage arrays 105. The storage arrays105 can be similarly distributed across multiple data centers.

Although in some embodiments certain commands used by the host devices102 to communicate with the storage arrays 105 illustratively compriseSCSI commands, other types of commands and command formats can be usedin other embodiments. For example, some embodiments can implement IOoperations utilizing command features and functionality associated withNVM Express (NVMe), as described in the NVMe Specification, Revision1.3, May 2017, which is incorporated by reference herein. Other storageprotocols of this type that may be utilized in illustrative embodimentsdisclosed herein include NVMe over Fabric, also referred to as NVMeoF,and NVMe over Transmission Control Protocol (TCP), also referred to asNVMe/TCP.

The storage arrays 105-1 and 105-2 are illustratively arranged in anactive-active configuration, although other storage configurations canbe used in other embodiments. In an example of an active-activeconfiguration that may be used, data stored in one of the storage arrays105 is replicated to the other one of the storage arrays 105 utilizing asynchronous replication process. Such data replication across themultiple storage arrays 105 can be used to facilitate failure recoveryin the system 100. One of the storage arrays 105 may therefore operateas a production storage array relative to the other storage array whichoperates as a backup or recovery storage array. Examples ofactive-active configurations include “metro” or “stretched” highavailability storage array configurations. The term “active-activeconfiguration” as used herein is therefore intended to be broadlyconstrued.

The storage arrays 105-1 and 105-2 are therefore assumed to beconfigured to participate in a replication process, such as asynchronous replication process. In accordance with one type ofsynchronous replication process, a given one of the host devices 102writes data to one of the storage arrays 105, and that host devicereceives an acknowledgement of success only after the data has beensuccessfully written to both of the storage arrays 105. For example, ifthe host device directs a write to the first storage array 105-1, thatstorage array mirrors the write to the second storage array 105-2 andreceives an acknowledgement of success back from the second storagearray 105-2. The first storage array 105-1 then responds back to thehost device with an acknowledgement of success.

The synchronous replication process is therefore configured to mirrordata writes from one or more of the host devices 102 to both of thestorage arrays 105. Other types of replication processes may be used inother embodiments.

For example, a “replication process” as that term is broadly used hereinmay include both asynchronous and synchronous replication modes as wellas support for concurrent operation of such modes and separate operationof the individual modes. It is also possible in some embodiments that agiven replication process implemented using storage arrays 105 maycomprise only synchronous replication or only asynchronous replication,instead of multiple distinct replication modes.

It is assumed that the storage controllers 108 of the respective storagearrays 105 each comprise replication control logic and a snapshotgenerator. The replication control logic illustratively controlsperformance of the above-noted synchronous replication process, or otherreplication processes in other embodiments. The snapshot generator isused to generate snapshots of one or more storage volumes that aresubject to synchronous replication in conjunction with active-activestorage clustering. Again, other types of storage configurations can beused in other embodiments.

The snapshots generated by the storage controllers 108 of the storagearrays 105 illustratively comprise respective point-in-time (PIT)replicas of the storage volumes. Multiple snapshots generated over timefor a given storage volume can collectively comprise a “snapshot group”and information characterizing those snapshots in some embodiments isstored in the form of a snapshot tree or other arrangement of one ormore data structures suitable for storing information characterizing asnapshot group. In some embodiments, a snapshot tree for a storagevolume is configured to add a new node each time a new snapshot isgenerated for that storage volume. The term “snapshot” as used herein isintended to be broadly construed, and in some embodiments may encompassa complete PIT replica or other types of information characterizing thestate of a given storage volume at a particular time.

A given storage volume designated for synchronous replication betweenstorage arrays 105 in the system 100 illustratively comprises a set ofone or more LUNs or other storage volumes of the storage arrays 105.Each such LUN or other storage volume is assumed to comprise at least aportion of a physical storage space of one or more of the storagedevices 106 of the corresponding storage arrays 105.

The host devices 102 comprise respective sets of IO queues 110-1 and110-2, and respective MPIO drivers 112-1 and 112-2. The MPIO drivers 112collectively comprise a multi-path layer of the host devices 102. Themulti-path layer provides automated path selection functionality usingrespective instances of path selection logic 114-1 and 114-2 implementedwithin the MPIO drivers 112.

The MPIO drivers 112 may comprise, for example, otherwise conventionalMPIO drivers, such as PowerPath® drivers from Dell EMC, suitablymodified in the manner disclosed herein to provide functionality forpath selection modification. Other types of MPIO drivers from otherdriver vendors may be suitably modified to incorporate functionality forpath selection modification as disclosed herein.

The MPIO driver 112-1 is configured to select IO operations from itscorresponding set of IO queues 110-1 for delivery to the storage arrays105 over the network 104. The sources of the IO operations stored in theset of IO queues 110-1 illustratively include respective processes ofone or more applications executing on the host device 102-1. Other typesof sources of IO operations may be present in a given implementation ofsystem 100.

The paths over which the IO operations are sent from the host device102-1 to the storage arrays 105 illustratively comprise paths associatedwith respective initiator-target pairs, with each initiator comprising ahost bus adaptor (HBA) or other initiating entity of the host device102-1 and each target comprising a storage array port or other targetedentity corresponding to one or more of the storage devices 106 of thestorage arrays 105. As noted above, the storage devices 106 of thestorage arrays 105 illustratively comprise LUNs or other types oflogical storage devices.

For example, in selecting particular ones of the paths for delivery ofthe IO operations to the storage arrays 105, the path selection logic114-1 of the MPIO driver 112-1 illustratively implements a pathselection algorithm that selects particular ones of the paths at leastin part as a function of path information such as host device HBA andstorage array port, with the path selection algorithm being configuredto balance the IO operations over the paths or to achieve other loadbalancing or performance goals.

Selecting a particular one of multiple available paths for delivery of aselected one of the IO operations of the set of IO queues 110-1 is moregenerally referred to herein as “path selection.” Path selection as thatterm is broadly used herein can in some cases involve both selection ofa particular IO operation and selection of one of multiple possiblepaths for accessing a corresponding logical device of one of the storagearrays 105. The corresponding logical device illustratively comprises aLUN or other logical storage volume to which the particular IO operationis directed.

A given retry of a failed IO operation under such a path selectionalgorithm can select a path having a different host device HBA andstorage array port for a given retry than that of the path selected forthe original failed IO operation.

The paths between the host devices 102 and the storage arrays 105 canchange over time. For example, the addition of one or more new pathsfrom host device 102-1 to the storage arrays 105 or the deletion of oneor more existing paths from the host device 102-1 to the storage arrays105 may result from respective addition or deletion of at least aportion of the storage devices 106 of the storage arrays 105. Additionor deletion of paths can also occur as a result of zoning and maskingchanges or other types of storage system reconfigurations performed by astorage administrator or other user.

In some embodiments, paths are added or deleted in conjunction withaddition of a new storage array or deletion of an existing storage arrayfrom a storage system that includes multiple storage arrays, possibly inconjunction with configuration of the storage system for at least one ofa migration operation and a replication operation.

In these and other situations, path discovery scans may be repeated asneeded in order to discover the addition of new paths or the deletion ofexisting paths.

A given path discovery scan can be performed utilizing knownfunctionality of conventional MPIO drivers, such as PowerPath® drivers.

The path discovery scan in some embodiments may be further configured toidentify one or more new LUNs or other logical storage volumesassociated with the one or more new paths identified in the pathdiscovery scan. The path discovery scan may comprise, for example, oneor more bus scans which are configured to discover the appearance of anynew LUNs that have been added to the storage arrays 105 as well todiscover the disappearance of any existing LUNs that have been deletedfrom the storage arrays 105.

The MPIO driver 112-1 in some embodiments comprises a user-space portionand a kernel-space portion. The kernel-space portion of the MPIO driver112-1 may be configured to detect one or more path changes of the typementioned above, and to instruct the user-space portion of the MPIOdriver 112-1 to run a path discovery scan responsive to the detectedpath changes. Other divisions of functionality between the user-spaceportion and the kernel-space portion of the MPIO driver 112-1 arepossible.

For each of one or more new paths identified in the path discovery scan,the host device 102-1 may be configured to execute a host registrationoperation for that path. The host registration operation for a given newpath illustratively provides notification to the corresponding one ofthe storage arrays 105 that the host device 102-1 has discovered the newpath.

As is apparent from the foregoing, MPIO driver 112-1 of host device102-1 is configured to control delivery of IO operations from the hostdevice 102-1 to the first and second storage arrays 105 over selectedpaths through the network 104.

Other host device components can additionally or alternatively performat least portions of controlling delivery of IO operations over selectedpaths, such as one or more host device processors or other control logicinstances. Illustrative embodiments are therefore not limited toarrangements in which MPIO drivers perform such delivery controlfunctions for IO operations. Moreover, terms such as “controllingdelivery” of an IO operation as used herein are intended to be broadlyconstrued so as to encompass, for example, selecting from a plurality ofpaths a particular path over which a particular IO operation is to besent to one of the storage arrays 105, and sending the IO operation overthat path.

In the FIG. 1 embodiment, the network 104 comprises first and secondswitch fabrics 104A and 104B through which the first and second hostdevices 102-1 and 102-2 are cross-connected to the first and secondstorage arrays 105-1 and 105-2 as shown. In a cross-connectedarrangement of this type, supporting active-active configuration of thestorage arrays 105 for the multiple host devices 102, use ofconventional path selection in the MPIO drivers 112 of host devices 102can lead to problems, as indicated previously. For example, if the MPIOdriver 112-1 were to implement the same path selection for all logicalstorage devices using a round robin path selection algorithm, it wouldsend at least different IO operations for each logical storage device todifferent ones of storage arrays 105. This can potentially undermine theperformance of various types of IO-based functionality of the individualstorage arrays 105, by in effect partially obscuring the actual IOpattern for a given logical storage device from each of the individualstorage arrays 105.

Such issues can arise in arrangements in which replication of a givenlogical storage device across storage arrays 105 involves “spoofing” oflogical storage device identifiers. For example, in accordance with suchspoofing, a replicated logical storage device will have the same deviceidentifier on both storage arrays 105. Accordingly, in embodiments ofthis type in which storage array information is embedded in the deviceidentifier, and the device on the second storage array 105-2 is spoofingthe device on the first storage array 105-1 by using its deviceidentifier, the storage array information embedded in the spoofed deviceidentifier will indicate the first storage array 105-1 and not thesecond storage array 105-2, even though the spoofing device is on thesecond storage array 105-2.

These and other issues are addressed in illustrative embodiments hereinby configuring the MPIO drivers 112 of the host devices 102 toincorporate active-active storage aware path selection functionality, aswill now be described in further detail.

The MPIO driver 112-1 in implementing at least portions of theactive-active storage aware path selection functionality of host device102-1 is further configured in illustrative embodiments to identify oneor more logical storage volumes or other logical storage devices thatare each accessible via at least first and second different ones of thepaths to respective ones of the first and second storage arrays 105, andto modify path selection for IO operations directed to the one or moreidentified logical storage devices relative to path selection for IOoperations directed to one or more other logical storage devices.

For example, modifying path selection in the host device 102-1 for theone or more identified logical storage devices each accessible viadifferent paths to both of the storage arrays 105 illustrativelycomprises modifying the path selection performed by path selection logic114-1 for a particular one of the one or more identified logical storagedevices, for at least a portion of a time period during which thatlogical storage device remains identified as being replicated across thestorage arrays 105.

The MPIO driver 112-1 in identifying one or more logical storage devicesthat are each accessible via at least first and second different ones ofthe paths to respective ones of the first and second storage arrays 105is illustratively configured to send commands on respective ones of thepaths over which a given logical storage device is accessible to thehost device 102-1. The MPIO driver 112-1 obtains, from at least one ofthe first and second storage arrays 105, information regarding the givenlogical storage device responsive to the commands, and determineswhether or not the given logical storage device is accessible via atleast first and second different ones of the paths to respective ones ofthe first and second storage arrays 105 based at least in part on theobtained information.

For example, the commands sent by the MPIO driver 112-1 over paths tothe first and second storage arrays for a given one of the logicalstorage devices illustratively comprise commands of a particular storageprotocol, such as a SCSI protocol or an NVMe protocol, that the hostdevice 102-1 utilizes to communicate with the first and second storagearrays 105.

Such commands may comprise respective inquiry page commands each ofwhich when received by one of the first and second storage arrays 105causes a designated page comprising logical storage device constituencyinformation to be returned by that storage array to the host device102-1.

In some embodiments, the obtained information for one of the commandssent by the MPIO driver 112-1 on one of the paths comprises anidentifier of the given logical storage device and an identifier of itscorresponding one of the first and second storage arrays 105.

Such information is illustratively obtained using a particular type ofinquiry page command, namely, a SCSI Inquiry (“Inq”) page 0x8B command.This command returns a device constituency page that includes both thedevice identifier and the storage array serial number or other storagearray identifier. Accordingly, the MPIO driver 112-1 by sending thiscommand on each of the paths associated with a given logical storagedevice can determine whether or not the device is accessible on multipledistinct storage arrays. Moreover, use of such a command overcomesissues associated with device spoofing, as the storage array identifieris obtained responsive to the command, in addition to the deviceidentifier. In this manner, even if device spoofing is used, the MPIOdriver 112-1 can still determine whether or not a given logical storagedevice is accessible on both of the storage arrays 105.

It should be noted that certain other types of inquiry page commands,such as a SCSI Inq page 0x83 which returns a potentially spoofed deviceidentifier without a separate storage array identifier, cannot be usedto definitively determine whether or not a given logical storage devicehas been replicated across the multiple storage arrays 105.

Illustrative embodiments therefore utilize inquiry page commands thatreveal the actual storage array identifier in addition to the deviceidentifier. Again, other types of commands can be used in otherembodiments.

The MPIO driver 112-1 can therefore identify the given logical storagedevice as one of the one or more logical storage devices that are eachaccessible via at least first and second different ones of the paths torespective ones of the first and second storage arrays 105 responsive tothe obtained information for one of the commands comprising anidentifier of the first storage array 105-1 and the obtained informationfor another one of the commands comprising an identifier of the secondstorage array 105-2.

For example, if the MPIO driver 112-1 obtains information for twodifferent commands, sent over different paths for the given logicalstorage device, that identifies different ones of the storage arrays105, the MPIO driver 112-1 knows that the given logical storage deviceis replicated over the storage arrays 105, illustratively in accordancewith the active-active configuration. The MPIO driver 112-1 responds tothis detected condition by modifying the manner in which it performspath selection for the given logical storage device. Similar operationsare used to identify other logical storage devices that are replicatedacross the first and second storage arrays 105 in accordance with theactive-active configuration.

It is to be appreciated that a wide variety of different types ofcommands or other types of communications between the host device 102-1and the storage arrays 105 can be used to allow the host device 102-1and its associated MPIO driver 112-1 to identify one or more logicalstorage devices that are replicated across the storage arrays 105, evenin the presence of spoofing of device identifiers.

In modifying path selection for IO operations directed to the one ormore identified logical storage devices, relative to path selection forIO operations directed to one or more other logical storage devices, theMPIO driver 112-1 illustratively separates a given one of the one ormore identified logical storage devices into at least first and secondportions, sends IO operations directed to the first portion over one ormore selected paths to the first storage array 105-1, and sends IOoperations directed to the second portion over one or more selectedpaths to the second storage array 105-2.

The use of paths to the first storage array 105-1 for the first portionof the given logical storage device and paths to the second storagearray 105-2 for the second portion of the given logical storage devicecan be varied, for example, with the relations between the portions andthe storage arrays 105 being reversed in other embodiments.

Path selection for the one or more other logical storage devices is notconfigured in this manner but instead, for example, treats each suchother logical storage device as a unitary device rather than as aseparated device in selecting paths for delivery of IO operationsdirected to that other logical storage device. For example, unmodifiedversions of load balancing algorithms or other path selectionalgorithms, such as round robin, least recently used (LRU), or othertypes of conventional path selection algorithms well known to thoseskilled in the art, may be used in selecting paths for IO operationsdirected to the logical storage devices that are not identified as beingreplicated across both storage arrays 105. Illustrative embodimentstherefore implement different types of path selection for logicalstorage devices that are identified as being replicated across thestorage arrays 105 than for the other logical storage devices that arenot identified as being replicated across the storage arrays 105.

In separating a given identified logical storage device into at leastfirst and second portions, the MPIO driver 112-1 illustrativelyseparates the given identified logical storage device into n contiguousportions each representing a corresponding fraction 1/n of a contiguouslogical address space of the identified logical storage device, where nis an even integer greater than or equal to two. For example, the MPIOdriver 112-1 can separate the given identified logical storage deviceinto two halves, such that n=2 and IO operations directed to the firsthalf are sent over paths to storage array 105-1 and IO operationsdirected to the second half are sent over paths to storage array 105-2.As another example, the MPIO driver 112-1 can separate the givenidentified logical storage device into four quarters, such that n=4 andIO operations directed to the first and third quarters are sent overpaths to storage array 105-1 and IO operations directed to the secondand fourth quarters are sent over paths to storage array 105-2. Numerousother separation arrangements are possible.

It is to be appreciated that terms such as “separating” and “separated”as used herein are intended to be broadly construed, and should not beviewed as requiring any type of physical separation of any logicalstorage device or its associated system components into differentportions. To the contrary, such separation of a given logical storagedevice is for purposes of path selection, with IO operations directed todifferent ones of the portions being sent by the MPIO driver 112-1 overselected paths to different ones of the storage arrays 105, so as toavoid adversely impacting data prefetching decisions, IO patternrecognition, automated storage tiering, machine learning and othersimilar IO-based functionality in the individual storage arrays 105,which might otherwise lead to degraded IO processing performance.

A wide variety of other logical storage device separation arrangementscan be used in other embodiments. A logical storage device that is notseparated in this manner is treated as what is referred to herein as a“unitary” or non-separated device for purposes of path selection. TheMPIO driver 112-1 illustratively provides different types of pathselection for separated devices than it does for non-separated devices.

In some embodiments, the MPIO driver 112-1 in modifying path selectionfor IO operations directed to the one or more identified logical storagedevices relative to path selection for IO operations directed to one ormore other logical storage devices more particularly determines aparticular portion of a given one of the one or more identified logicalstorage devices that is experiencing a relatively high level of IOactivity relative to one or more other portions of the given identifiedlogical storage device. The particular portion is illustratively one ofthe above-noted portions into which the given identified logical storagedevice is separated, such as the first half of such a device. The MPIOdriver 112-1 then separates the particular portion into at least firstand second sub-portions, sends IO operations directed to the firstsub-portion over one or more selected paths to the first storage array105-1, and sends IO operations directed to the second sub-portion overone or more selected paths to the second storage array 105-2. Asindicated previously, path selection for the one or more other logicalstorage devices is not configured in this manner but instead, forexample, treats each such other logical storage device as a unitarydevice rather than as a separated device.

Although in the present embodiment and other embodiments herein MPIOdrivers are used to perform path selection modification in conjunctionwith active-active storage aware path selection, this is by way ofillustrative example only, and other host device components canalternatively implement at least portions of such path selectionmodification functionality. Accordingly, path selection modificationfunctionality in some embodiments can be distributed across multiplehost device components, possibly including MPIO drivers in combinationwith other host device components such as host device processors andassociated control logic instances.

As described above, the MPIO driver 112-1 in the FIG. 1 embodiment isconfigured to identify one or more logical storage volumes or otherlogical storage devices that are each accessible via paths to both ofthe storage arrays 105. The MPIO driver 112-1 illustratively maintainsone or more data structures that specify each path or set of pathsassociated with a given logical storage device to which IO operationsmay be directed by the MPIO driver 112-1, including informationcharacterizing the particular HBA and storage array port that are therespective initiator and target for each such path.

The above-noted data structures may more particularly comprise storagearray objects that include “inventories” of storage devices of theircorresponding storage arrays, with such objects being maintained by theMPIO driver 112-1. For example, a first data structure illustrativelycomprises a first object specifying a first set of paths between thehost device 102-1 and at least one of the first and second storagearrays 105, and a second data structure comprises a second objectspecifying a second set of paths between the host device 102-1 and atleast one of the first and second storage arrays 105. In someembodiments, at least one of the first object and the second objectcomprises a federated object that specifies paths to both the first andthe second storage arrays for a paired logical device that is identifiedby the MPIO driver 112-1 as a single logical device but has separatecorresponding logical devices on the respective first and second storagearrays 105.

Other types and arrangements of data structures maintained by the MPIOdriver 112-1 can be used in identifying one or more logical storagedevices that are each accessible via paths to both of the storage arrays105.

In some embodiments, the MPIO driver 112-1 in modifying path selectionin the host device 102-1 for the one or more identified logical storagedevices is more particularly configured, for example, to modify theoperation of a load balancing algorithm or other path selectionalgorithm implemented by the path selection logic 114-1.

For example, the MPIO driver 112-1 can illustratively modify theoperation of a load balancing algorithm that it uses in path selectionin the path selection logic 114-1 based at least in part on the one ormore identified logical storage devices that are each accessible viapaths to both of the storage arrays 105.

The MPIO driver 112-1 can also detect for at least one of the one ormore identified logical storage devices a change in its configuration.For example, a given one of the one or more identified logical storagedevices that are each accessible via paths to both of the storage arrays105 can have its configuration altered so that it is no longeraccessible via paths to both of the storage arrays 105. Responsive todetection of such a change, the MPIO driver 112-1 can restore themodified path selection in the host device 102-1 for that logicalstorage device to its state prior to the modification. Accordingly, byway of illustrative example, after all of the logical storage devicespreviously identified as being accessible via paths to both of thestorage arrays 105 are no longer accessible via paths to both of thestorage arrays 105, any corresponding modifications in path selectionimplemented by the MPIO driver 112-1 can be reversed, to thereby restorenormal path selection functionality.

The above-described functions associated with path selectionmodification in the MPIO driver 112-1 are illustratively carried out atleast in part under the control of its path selection logic 114-1. Forexample, the path selection logic 114-1 is illustratively configured tocontrol performance of the steps of the flow diagram to be describedbelow in conjunction with FIG. 2. In other embodiments, the FIG. 2process can be performed at least in part by other host devicecomponents, such as by one or more host device processors and/orassociated control logic instances.

It is assumed that the other MPIO driver 112-2 is configured in a mannersimilar to that described above and elsewhere herein for the first MPIOdriver 112-1. The MPIO driver 112-2 is therefore similarly configured toselect IO operations from its corresponding one of the sets of IO queues110 for delivery to the storage arrays 105 over the network 104 and toperform the disclosed path selection modification functionality.Accordingly, path selection modification functionality described abovein the context of the first MPIO driver 112-1 is assumed to be similarlyperformed by the other MPIO driver 112-2.

The MPIO drivers 112 in some embodiments can include well-known MPIOfunctionality such as that described in “Dell EMC SC Series Storage andMicrosoft Multipath I/O,” Dell EMC, CML1004, July 2018, which isincorporated by reference herein. Such conventional MPIO functionalityis suitably modified in illustrative embodiments disclosed herein tosupport path selection modification.

It is to be appreciated that the above-described features of system 100and other features of other illustrative embodiments are presented byway of example only, and should not be construed as limiting in any way.Accordingly, different numbers, types and arrangements of systemcomponents such as host devices 102, network 104, storage arrays 105,storage devices 106, sets of IO queues 110, MPIO drivers 112 andinstances of path selection logic 114 can be used in other embodiments.

It should also be understood that the particular sets of modules andother components implemented in the system 100 as illustrated in FIG. 1are presented by way of example only. In other embodiments, only subsetsof these components, or additional or alternative sets of components,may be used, and such components may exhibit alternative functionalityand configurations.

The operation of the information processing system 100 will now bedescribed in further detail with reference to the flow diagram of theillustrative embodiment of FIG. 2. The process as shown includes steps200 through 208, and is suitable for use in the system 100 but is moregenerally applicable to other types of systems comprising one or morehost devices and at least first and second storage systems. The storagesystems in this embodiment are assumed to more particularly compriserespective first and second storage arrays each comprising a pluralityof storage devices. The storage devices of the first and second storagearrays are assumed to include logical storage devices such as LUNs orother logical storage volumes.

The steps of the FIG. 2 process are illustratively performed primarilyby or under the control of an MPIO driver of a given host device, suchas the MPIO driver 112-1 of the first host device 102-1 of system 100,although other arrangements of system components can perform at leastportions of one or more of the steps in other embodiments. Thefunctionality of the FIG. 2 process is illustratively performed inconjunction with a load balancing algorithm or other path selectionalgorithm executed by the path selection logic 114-1.

In step 200, the MPIO driver performs path selection while monitoringfor one or more logical storage devices that are replicated across firstand second storage arrays arranged in an active-active configuration.

In step 202, a determination is made by the MPIO driver as to whether ornot one or more such replicated logical storage devices have beenidentified. If at least one such logical storage device has beenidentified, the process moves to step 204, and otherwise returns to step200 to continue performing path selection while monitoring for anylogical storage devices that are replicated across the first and secondstorage arrays.

In step 204, which is reached responsive to detection of at least onelogical storage device that is replicated across the first and secondstorage arrays, the MPIO driver uses modified path selection for the oneor more identified logical storage devices. Although not explicitlyindicated in step 204, the MPIO driver also monitors for changes inconfiguration of the one or more identified logical storage devices.

In step 206, a determination is made by the MPIO driver as to whether ornot any of the one or more identified logical storage devices are nolonger replicated across the first and second storage arrays, but areinstead, for example, now accessible on only one of the storage arrays.This may be due, for example, to updated zoning and masking or othertypes of storage system reconfigurations subsequent to theidentification of the one or more logical storage devices in conjunctionwith steps 200 and 202. If the one or more identified logical storagedevices are no longer replicated across the first and second storagearrays, the process moves to step 208, and otherwise returns to step 204to continue using the modified path selection for the one or moreidentified logical storage devices, while also monitoring for changes inconfiguration of the one or more identified logical storage devices.

In step 208, which is reached responsive to at least one of the one ormore identified logical storage devices no longer being replicatedacross the first and second storage arrays, the MPIO driver resumes itsnormal path selection for any such previously identified logical storagedevice that is no longer replicated across the first and second storagearrays. The process then returns to step 200 as indicated.

Different instances of steps 204, 206 and 208 can be separatelyperformed for each of the logical storage devices identified as beingreplicated across the first and second storage arrays in conjunctionwith steps 200 and 202.

The various steps of the FIG. 2 process are illustratively shown asbeing performed serially, but certain steps can at least partiallyoverlap with other steps.

Also, multiple additional instances of the FIG. 2 process can beperformed in respective ones of one or more additional host devices thatshare the first and second storage arrays. As another example, differentinstances of the FIG. 2 process can be performed for each of a pluralityof different logical storage devices, with each such instance separatelydetecting replication of a corresponding logical storage device acrossthe first and second storage arrays, and controlling path selection withrespect to that identified logical storage device.

The particular processing operations and other system functionalitydescribed in conjunction with the flow diagram of FIG. 2 are presentedby way of illustrative example only, and should not be construed aslimiting the scope of the disclosure in any way. Alternative embodimentscan use other types of processing operations involving host devices,storage systems and path selection modification functionality. Forexample, the ordering of the process steps may be varied in otherembodiments, or certain steps may be performed at least in partconcurrently with one another rather than serially. Also, one or more ofthe process steps may be repeated periodically, or multiple instances ofthe process can be performed in parallel with one another in order toimplement a plurality of different path selection modificationarrangements within a given information processing system.

Functionality such as that described in conjunction with the flowdiagram of FIG. 2 can be implemented at least in part in the form of oneor more software programs stored in memory and executed by a processorof a processing device such as a computer or server. As will bedescribed below, a memory or other storage device having executableprogram code of one or more software programs embodied therein is anexample of what is more generally referred to herein as a“processor-readable storage medium.”

Referring now to FIG. 3, another illustrative embodiment is shown. Inthis embodiment, an information processing system 300 comprisesapplication processes 311, path selection logic 314 and replicationcontrol logic 321. The system 300 is configured in accordance with alayered system architecture that illustratively includes a host deviceprocessor layer 330, an MPIO layer 332, an HBA layer 334, a switchfabric layer 336, a storage array port layer 338 and a storage arrayprocessor layer 340. As illustrated in the figure, the host deviceprocessor layer 330, the MPIO layer 332 and the HBA layer 334 areassociated with one or more host devices, the switch fabric layer 336 isassociated with one or more SANs or other types of networks, and thestorage array port layer 338 and storage array processor layer 340 areassociated with one or more storage arrays (“SAs”).

The system 300 in this embodiment implements path selection modificationin conjunction with identification of one or more logical storagedevices that are replicated across first and second storage arrays,illustratively arranged in an active-active configuration relative toone another, in a manner similar to that described elsewhere herein. Theapplication processes 311 are illustratively running in one or more hostdevice processors of the host device processor layer 330. The pathselection modification functionality in this embodiment is assumed to becontrolled at least in part by path selection logic 314 of the MPIOlayer 332, although other arrangements are possible.

The MPIO layer 332 is an example of what is also referred to herein as amulti-path layer, and comprises one or more MPIO drivers implemented inrespective host devices. Each such MPIO driver illustratively comprisesan instance of path selection logic 314 configured to implementfunctionality for path selection modification in conjunction withidentification of one or more logical storage devices that arereplicated across first and second storage arrays as previouslydescribed. Additional or alternative layers and path selection logicarrangements can be used in other embodiments.

The replication control logic 321 implemented in the storage arrayprocessor layer 340 controls the active-active configuration of a givenpair of storage arrays, or other types of replication arrangementsimplemented in the system 300. For example, the replication controllogic 321 can include functionality for carrying out a synchronousreplication process between first and second storage arrays in theactive-active configuration. It is also possible in some embodimentsthat the replication control logic 321 can include multiple distinctreplication control logic instances for respective ones of a pluralityof storage arrays of the system 300. Although not explicitly shown inthe figure, additional replication control logic is illustrativelyimplemented in the host device processor layer 330, or elsewhere in thesystem 300, such as in the MPIO layer 332.

In the system 300, path selection logic 314 is configured to selectdifferent paths for sending IO operations from a given host device to astorage array. These paths as illustrated in the figure include a firstpath from a particular HBA denoted HBA1 through a particular switchfabric denoted SF1 to a particular storage array port denoted PORT1, anda second path from another particular HBA denoted HBA2 through anotherparticular switch fabric denoted SF2 to another particular storage arrayport denoted PORT2.

These two particular paths are shown by way of illustrative exampleonly, and in many practical implementations there will typically be amuch larger number of paths between the one or more host devices and theone or more storage arrays, depending upon the specific systemconfiguration and its deployed numbers of HBAs, switch fabrics andstorage array ports. For example, each host device in the FIG. 3embodiment can illustratively have a particular number and type of pathsto a shared storage array, or alternatively different ones of the hostdevices can have different numbers and types of paths to the storagearray.

The path selection logic 314 of the MPIO layer 332 in this embodimenttherefore selects paths for delivery of IO operations to the one or morestorage arrays having the storage array ports of the storage array portlayer 338. In selecting the paths in conjunction with identification ofone or more logical storage devices that are replicated across first andsecond storage arrays, the path selection logic 314 implements adifferent path selection for IO operations directed to the one or moreidentified logical storage devices than it does for IO operationsdirected to one or more other logical storage devices, as describedelsewhere herein. For example, the path selection logic 314 can modify aload balancing algorithm or other path selection algorithm normally usedto select the paths for delivery of IO operations such that the pathsare selected in a different manner for the one or more identifiedlogical storage devices than for one or more other logical storagedevices that are not so identified.

Accordingly, in this embodiment the host devices of system 300 throughtheir respective MPIO drivers and respective instances of path selectionlogic 314 provide functionality for path selection modification inconjunction with identification of one or more logical storage devicesthat are replicated across multiple storage arrays, possibly withinvolvement of other host device or system components.

Some implementations of the system 300 can include a relatively largenumber of host devices (e.g., 1000 or more host devices), although asindicated previously different numbers of host devices, and possiblyonly a single host device, may be present in other embodiments. Each ofthe host devices is typically allocated with a sufficient number of HBAsto accommodate predicted performance needs. In some cases, the number ofHBAs per host device is on the order of 4, 8 or 16 HBAs, although othernumbers of HBAs could be allocated to each host device depending uponthe predicted performance needs. A typical storage array may include onthe order of 128 ports, although again other numbers can be used basedon the particular needs of the implementation. The number of hostdevices per storage array port in some cases can be on the order of 10host devices per port. The HBAs of the host devices are assumed to bezoned and masked to the storage array ports in accordance with thepredicted performance needs, including user load predictions.

A given host device of system 300 can be configured to initiate anautomated path discovery process to discover new paths responsive toupdated zoning and masking or other types of storage systemreconfigurations performed by a storage administrator or other user. Forcertain types of host devices, such as host devices using particularoperating systems such as Windows, ESX or Linux, automated pathdiscovery via the MPIO drivers of a multi-path layer is typicallysupported. Other types of host devices using other operating systemssuch as AIX in some implementations do not necessarily support suchautomated path discovery, in which case alternative techniques can beused to discover paths.

Again, different instances of the above-described path selectionmodification process can be performed by different MPIO drivers indifferent host devices.

Another example of a path selection modification process implementedutilizing a multi-path layer such as MPIO layer 332 of the FIG. 3embodiment will now be described in more detail. Such a process utilizespath selection logic of one or more host devices to provide modifiedpath selection responsive to identification of one or more logicalstorage devices that are replicated across first and second storagearrays that are arranged in an active-active configuration relative toone another.

As indicated previously, performance issues can arise when the same typeof path selection is applied both to logical storage devices that arereplicated across multiple storage arrays and to logical storage devicesthat are not replicated across the multiple storage arrays but areinstead accessible on only one of the storage arrays.

For example, path selection logic in conventional host deviceimplementations can adversely impact data prefetching decisions, IOpattern recognition, automated storage tiering, machine learning andother similar IO-based functionality in the individual storage arrays,potentially leading to degraded IO processing performance.

Illustrative embodiments disclosed herein advantageously overcome theseand other drawbacks of conventional arrangements, for example, thoughpath selection modification in a multi-path layer of the host device, aswill now be further described in the context of the present example. Thepresent example more particularly comprises an algorithm that isillustratively performed by an MPIO driver of a multi-path layer of ahost device and includes the following steps:

1. The MPIO driver issues an Inq page 0x8B command on each path forwhich a given logical storage device is visible to the host device. Asindicated previously, such a command returns a device constituency pagethat includes an identifier of the given logical storage device as wellas a corresponding storage array identifier, illustratively a storagearray serial number. Accordingly, if different ones of the commands sentover respective different ones of the paths for the given logicalstorage device return different storage array identifiers, the MPIOdriver identifies that logical storage device as being accessible viathe different paths to the multiple storage arrays.

2. The MPIO driver denotes the storage array having the lowest returnedserial number for the identified logical storage device as “array-1” andthe other storage array for which a serial number was returned for theidentified logical storage device as “array-2.” This assignment can bereversed in other embodiments.

3. The MPIO driver separates the identified logical storage device intofirst and second halves, illustratively an upper portion and a lowerportion of the logical address space of the device. As indicatedpreviously, other types of separation of an identified logical storagedevice into multiple portions can be used.

4. The MPIO driver alters its path selection for the identified logicalstorage device such that all IO operations directed to logical addressesin the first half of the device are sent to array-1, and all IOoperations directed to logical addresses the second half of the deviceare sent to array-2.

Similar operations are performed for each of one or more additionallogical storage devices of the system.

The above example algorithm avoids adverse impacts to data prefetchingdecisions, IO pattern recognition, automated storage tiering, machinelearning and other similar IO-based functionality in the individualstorage arrays, while also ensuring that none of the storage arrays areinadvertently overloaded with all of the IO operations for a givenlogical storage device, for example, in the case of device spoofingperformed in conjunction with the active-active configuration of thestorage arrays.

Other types of separation of an identified logical storage device can beused, instead of the separation into upper and lower halves as describedabove. For example, the device can be separated into four portions, eachcorresponding to a different contiguous range of logical addresses ofthe logical storage device, with the IO operations directed to logicaladdresses within a given one of the portions always being sent to thesame storage array. Thus, IO operations directed to logical addresses inthe first quarter are sent to array-1, IO operations directed to logicaladdresses in the second quarter are sent to array-2, IO operationsdirected to logical addresses in the third quarter are sent to array-1,and so on.

The above example algorithm can be further enhanced to take into accountIO locality. For example, if the first half of the device has a higherIO activity than the second half of the device, which is likely in thecase of a growing database, then the MPIO can further enhance thedivision resolution as follows:

1. The MPIO driver tracks the logical address range currently beingused, and divides this portion of the logical storage device intosub-portions. For example, the MPIO driver may identify that a portioncorresponding to the first quarter of the device is currently subject toelevated IO activity relative to other portions of the device, andseparate the identified portion into two sub-portions, eachcorresponding to ⅛ of the device.

2. The MPIO will send IO operations directed to logical addresses in thefirst sub-portion (e.g., the first ⅛ of the device) to array-1 and willsend IO operations directed to logical addresses in the secondsub-portion (e.g., the second ⅛ of the device) to array-2.

Again, numerous other separations of a given logical storage device intoportions and sub-portions are possible, with associated adjustments inpath selection for the different portions or sub-portions to ensure thatIO operations directed to those portions or sub-portions are sent overpaths to the appropriate storage arrays.

These particular steps are illustrative only, and additional oralternative steps can be used in other embodiments. Also, although shownas being performed serially, one or more of the steps may each at leastpartially overlap with other ones of the steps.

Illustrative embodiments can be implemented, for example, in one or moreMPIO drivers of one or more host devices, with such MPIO driverscollectively providing a multi-path layer of the host devices.

For example, some embodiments are implemented though modification ofotherwise conventional multi-pathing software, such as PowerPath®drivers commercially available from Dell EMC. Other embodiments can beimplemented in other MPIO drivers from other multi-pathing softwarevendors.

Moreover, other host device components, such as logic instances and/orhost processors, can additionally or alternatively be used.

The process in the present example advantageously provides proactiveadjustments in a path selection algorithm responsive to identificationof one or more logical storage devices that are replicated across firstand second storage arrays, in a manner that improves overall IOprocessing performance.

The process is illustratively performed by one or more MPIO drivers andassociated path selection logic instances of a multi-path layer of agiven host device. A similar process is assumed to be performed on anyrespective other host devices.

Other types of path selection modification involving alteration of loadbalancing logic or other path selection logic can be implemented in oneor more host devices in other embodiments in conjunction withidentification of one or more logical storage devices that arereplicated across first and second storage arrays in an active-activeconfiguration.

Some embodiments include only a single host device, although multiplehost devices are used in illustrative embodiments. For example, a singlehost device can be connected to two storage arrays that are arranged inan active-active configuration.

Also, it should be noted that the host devices in a given embodimentneed not be in an active-active configuration. For example, multiplehost devices can be arranged in a cluster and the host devices can bearranged in active-passive configurations, active-active configurations,or combinations thereof.

The particular path selection modification arrangements described aboveare presented by way of illustrative example only. Numerous alternativearrangements of these and other features can be used in implementingpath selection modification in other embodiments.

The illustrative embodiments disclosed herein can provide a number ofsignificant advantages relative to conventional arrangements.

For example, some embodiments configure a host device to include pathselection modification functionality for one or more logical storagedevices that are identified as being replicated across multiple storagesystems, such as first and second storage systems arranged in anactive-active configuration.

Illustrative embodiments can provide active-active storage aware pathselection in a host device. The path selection is illustratively“active-active storage aware” in that it can determine whether or not agiven logical storage volume or other logical storage device isreplicated across multiple storage arrays in an active-activeconfiguration, and adjust the selection process accordingly so as toimprove performance.

In some embodiments, the active-active storage aware path selection isimplemented in a multi-path layer that comprises at least one MPIOdriver configured to process IO operations of at least one host devicethat communicates with multiple storage arrays or other types of storagesystems.

The multi-path layer in such arrangements can be configured, forexample, to detect particular logical storage volumes or other logicalstorage devices that are replicated across multiple storage arrays orother storage systems in an active-active configuration, such that themulti-path layer thereby becomes “aware” of the active-activeconfiguration for those storage devices, and to modify the manner inwhich it performs path selection so as to provide enhanced loadbalancing between the multiple storage arrays under such conditions.

For example, the active-active storage aware path selection can beadvantageously configured in some embodiments to ensure that pathselection in the host device does not adversely impact data prefetchingdecisions, IO pattern recognition, automated storage tiering, machinelearning or other similar IO-based functionality in the individualstorage arrays. As a result, improved performance in processing of IOoperations by the storage arrays is achieved.

The disclosed functionality can be implemented using a wide variety oftypes of host devices each configured to interact with multiple distinctstorage arrays or other types of storage systems.

These and other arrangements are advantageously configured to providepath selection modification for efficient load balancing even in thepresence of substantial path changes such as those that may result whenpaths are added or deleted as a result of zoning and masking changes orother types of storage system reconfigurations performed by a storageadministrator or other user.

It is to be appreciated that the particular advantages described aboveare associated with particular illustrative embodiments and need not bepresent in other embodiments. Also, the particular types of informationprocessing system features and functionality as illustrated in thedrawings and described above are exemplary only, and numerous otherarrangements may be used in other embodiments.

It was noted above that portions of an information processing system asdisclosed herein may be implemented using one or more processingplatforms. Illustrative embodiments of such platforms will now bedescribed in greater detail. These and other processing platforms may beused to implement at least portions of other information processingsystems in other embodiments. A given such processing platform comprisesat least one processing device comprising a processor coupled to amemory.

One illustrative embodiment of a processing platform that may be used toimplement at least a portion of an information processing systemcomprises cloud infrastructure including virtual machines implementedusing a hypervisor that runs on physical infrastructure. The cloudinfrastructure further comprises sets of applications running onrespective ones of the virtual machines under the control of thehypervisor. It is also possible to use multiple hypervisors eachproviding a set of virtual machines using at least one underlyingphysical machine. Different sets of virtual machines provided by one ormore hypervisors may be utilized in configuring multiple instances ofvarious components of the system.

These and other types of cloud infrastructure can be used to providewhat is also referred to herein as a multi-tenant environment. One ormore system components such as virtual machines, or portions thereof,are illustratively implemented for use by tenants of such a multi-tenantenvironment.

Cloud infrastructure as disclosed herein can include cloud-based systemssuch as AWS, GCP and Microsoft Azure. Virtual machines provided in suchsystems can be used to implement a fast tier or other front-end tier ofa multi-tier storage system in illustrative embodiments. A capacity tieror other back-end tier of such a multi-tier storage system can beimplemented using one or more object stores such as Amazon S3, GCP CloudStorage, and Microsoft Azure Blob Storage.

In some embodiments, the cloud infrastructure additionally oralternatively comprises a plurality of containers illustrativelyimplemented using respective operating system kernel control groups ofone or more container host devices. For example, a given container ofcloud infrastructure illustratively comprises a Docker container orother type of LXC implemented using a kernel control group. Thecontainers may run on virtual machines in a multi-tenant environment,although other arrangements are possible. The containers may be utilizedto implement a variety of different types of functionality within thesystem 100. For example, containers can be used to implement respectivecompute nodes or storage nodes of a cloud-based system. Again,containers may be used in combination with other virtualizationinfrastructure such as virtual machines implemented using a hypervisor.

Another illustrative embodiment of a processing platform that may beused to implement at least a portion of an information processing systemcomprises a plurality of processing devices which communicate with oneanother over at least one network. The network may comprise any type ofnetwork, including by way of example a global computer network such asthe Internet, a WAN, a LAN, a satellite network, a telephone or cablenetwork, a cellular network, a wireless network such as a WiFi or WiMAXnetwork, or various portions or combinations of these and other types ofnetworks.

Each processing device of the processing platform comprises a processorcoupled to a memory. The processor may comprise a microprocessor, amicrocontroller, an application-specific integrated circuit (ASIC), afield-programmable gate array (FPGA), a graphics processing unit (GPU)or other type of processing circuitry, as well as portions orcombinations of such circuitry elements. The memory may comprise randomaccess memory (RAM), read-only memory (ROM), flash memory or other typesof memory, in any combination. The memory and other memories disclosedherein should be viewed as illustrative examples of what are moregenerally referred to as “processor-readable storage media” storingexecutable program code of one or more software programs.

Articles of manufacture comprising such processor-readable storage mediaare considered illustrative embodiments. A given such article ofmanufacture may comprise, for example, a storage array, a storage diskor an integrated circuit containing RAM, ROM, flash memory or otherelectronic memory, or any of a wide variety of other types of computerprogram products. The term “article of manufacture” as used hereinshould be understood to exclude transitory, propagating signals.

Also included in the processing device is network interface circuitry,which is used to interface the processing device with the network andother system components, and may comprise conventional transceivers.

As another example, portions of a given processing platform in someembodiments can comprise converged infrastructure such as VxRail™,VxRack™, VxRack™ FLEX, VxBlock™ or Vblock® converged infrastructure fromDell EMC.

Again, these particular processing platforms are presented by way ofexample only, and other embodiments may include additional oralternative processing platforms, as well as numerous distinctprocessing platforms in any combination, with each such platformcomprising one or more computers, servers, storage devices or otherprocessing devices.

It should therefore be understood that in other embodiments differentarrangements of additional or alternative elements may be used. At leasta subset of these elements may be collectively implemented on a commonprocessing platform, or each such element may be implemented on aseparate processing platform.

Also, numerous other arrangements of computers, servers, storage devicesor other components are possible in an information processing system asdisclosed herein. Such components can communicate with other elements ofthe information processing system over any type of network or othercommunication media.

As indicated previously, components of an information processing systemas disclosed herein can be implemented at least in part in the form ofone or more software programs stored in memory and executed by aprocessor of a processing device. For example, at least portions of thefunctionality of host devices 102, network 104 and storage arrays 105are illustratively implemented in the form of software running on one ormore processing devices. As a more particular example, the instances ofpath selection logic 114 may be implemented at least in part insoftware, as indicated previously herein.

It should again be emphasized that the above-described embodiments arepresented for purposes of illustration only. Many variations and otheralternative embodiments may be used. For example, the disclosedtechniques are applicable to a wide variety of other types ofinformation processing systems, utilizing other arrangements of hostdevices, networks, storage systems, storage arrays, storage devices,processors, memories, IO queues, MPIO drivers, path selection logic andadditional or alternative components. Also, the particularconfigurations of system and device elements and associated processingoperations illustratively shown in the drawings can be varied in otherembodiments. For example, a wide variety of different MPIO driverconfigurations and associated path selection modification arrangementscan be used in other embodiments. Moreover, the various assumptions madeabove in the course of describing the illustrative embodiments shouldalso be viewed as exemplary rather than as requirements or limitations.Numerous other alternative embodiments within the scope of the appendedclaims will be readily apparent to those skilled in the art.

What is claimed is:
 1. An apparatus comprising: at least one processingdevice comprising a processor coupled to a memory; said at least oneprocessing device being configured: to control delivery of input-outputoperations from a host device to at least first and second storagesystems over selected ones of a plurality of paths through a network,the first and second storage systems being arranged in an active-activeconfiguration relative to one another; to identify one or more logicalstorage devices that are each accessible via at least first and seconddifferent ones of the paths to respective ones of the first and secondstorage systems; and to modify path selection for input-outputoperations directed to the one or more identified logical storagedevices relative to path selection for input-output operations directedto one or more other logical storage devices.
 2. The apparatus of claim1 wherein modifying path selection for input-output operations directedto the one or more identified logical storage devices relative to pathselection for input-output operations directed to one or more otherlogical storage devices comprises: performing path selection for a givenone of the one or more identified logical storage devices in a mannerthat separates the given identified logical storage device into multipleportions and applies different path selection techniques to differentones of the portions; and performing path selection for a given one ofthe one or more other logical storage devices in a manner that treatsthe given other logical storage device as a unitary device and applies asingle path selection technique to that unitary device.
 3. The apparatusof claim 1 wherein the paths are associated with respectiveinitiator-target pairs, the initiators being implemented on the hostdevice and the targets being implemented on the first and second storagesystems, and wherein the initiators of the initiator-target pairscomprise respective host bus adaptors of the host device and the targetsof the initiator-target pairs comprise respective storage array ports ofthe first and second storage systems.
 4. The apparatus of claim 1wherein said at least one processing device comprises at least a portionof the host device.
 5. The apparatus of claim 4 wherein said at leastone processing device comprises a multi-path input-output driver of thehost device, with the multi-path input-output driver of the host devicebeing configured to control the delivery of the input-output operationsfrom the host device to the first and second storage systems over theselected ones of the plurality of paths through the network.
 6. Theapparatus of claim 5 wherein the multi-path input-output driver isfurther configured to perform at least a portion of the identifying oneor more logical storage devices, and the modifying of path selection forinput-output operations directed to the one or more identified logicalstorage devices.
 7. The apparatus of claim 1 wherein identifying one ormore logical storage devices that are each accessible via at least firstand second different ones of the paths to respective ones of the firstand second storage systems comprises: sending commands on respectiveones of the paths over which a given logical storage device isaccessible to the host device; obtaining from at least one of the firstand second storage systems information regarding the given logicalstorage device responsive to the commands; and determining whether ornot the given logical storage device is accessible via at least firstand second different ones of the paths to respective ones of the firstand second storage systems based at least in part on the obtainedinformation.
 8. The apparatus of claim 7 wherein the commands compriserespective commands of a storage protocol that the host device utilizesto communicate with the first and second storage systems.
 9. Theapparatus of claim 7 wherein the commands comprise respective inquirypage commands each of which when received by one of the first and secondstorage systems causes a designated page comprising logical storagedevice constituency information to be returned by that storage system tothe host device.
 10. The apparatus of claim 7 wherein the obtainedinformation for one of the commands sent on one of the paths comprisesan identifier of the given logical storage device and an identifier ofits corresponding one of the first and second storage systems.
 11. Theapparatus of claim 10 wherein the given logical storage device isidentified as one of the one or more logical storage devices that areeach accessible via at least first and second different ones of thepaths to respective ones of the first and second storage systemsresponsive to the obtained information for one of the commandscomprising an identifier of the first storage system and the obtainedinformation for another one of the commands comprising an identifier ofthe second storage system.
 12. The apparatus of claim 1 whereinmodifying path selection for input-output operations directed to the oneor more identified logical storage devices relative to path selectionfor input-output operations directed to one or more other logicalstorage devices comprises: separating a given one of the one or moreidentified logical storage devices into at least first and secondportions; sending input-output operations directed to the first portionover one or more selected paths to the first storage system; and sendinginput-output operations directed to the second portion over one or moreselected paths to the second storage system.
 13. The apparatus of claim12 wherein separating the given identified logical storage device intoat least first and second portions comprises separating the givenidentified logical storage device into n contiguous portions eachrepresenting a corresponding fraction 1/n of a contiguous logicaladdress space of the identified logical storage device, where n is aneven integer greater than or equal to two.
 14. The apparatus of claim 1wherein modifying path selection for input-output operations directed tothe one or more identified logical storage devices relative to pathselection for input-output operations directed to one or more otherlogical storage devices comprises: determining a particular portion of agiven one of the one or more identified logical storage devices that isexperiencing a relatively high level of input-output activity relativeto one or more other portions of the given identified logical storagedevice; separating the particular portion into at least first and secondsub-portions; sending input-output operations directed to the firstsub-portion over one or more selected paths to the first storage system;and sending input-output operations directed to the second sub-portionover one or more selected paths to the second storage system.
 15. Amethod comprising: controlling delivery of input-output operations froma host device to at least first and second storage systems over selectedones of a plurality of paths through a network, the first and secondstorage systems being arranged in an active-active configurationrelative to one another; identifying one or more logical storage devicesthat are each accessible via at least first and second different ones ofthe paths to respective ones of the first and second storage systems;and modifying path selection for input-output operations directed to theone or more identified logical storage devices relative to pathselection for input-output operations directed to one or more otherlogical storage devices; wherein the method is performed by at least oneprocessing device comprising a processor coupled to a memory.
 16. Themethod of claim 15 wherein identifying one or more logical storagedevices that are each accessible via at least first and second differentones of the paths to respective ones of the first and second storagesystems comprises: sending commands on respective ones of the paths overwhich a given logical storage device is accessible to the host device;obtaining from at least one of the first and second storage systemsinformation regarding the given logical storage device responsive to thecommands; and determining whether or not the given logical storagedevice is accessible via at least first and second different ones of thepaths to respective ones of the first and second storage systems basedat least in part on the obtained information.
 17. The method of claim 15wherein modifying path selection for input-output operations directed tothe one or more identified logical storage devices relative to pathselection for input-output operations directed to one or more otherlogical storage devices comprises: separating a given one of the one ormore identified logical storage devices into at least first and secondportions; sending input-output operations directed to the first portionover one or more selected paths to the first storage system; and sendinginput-output operations directed to the second portion over one or moreselected paths to the second storage system.
 18. A computer programproduct comprising a non-transitory processor-readable storage mediumhaving stored therein program code of one or more software programs,wherein the program code, when executed by at least one processingdevice comprising a processor coupled to a memory, causes said at leastone processing device: to control delivery of input-output operationsfrom a host device to at least first and second storage systems overselected ones of a plurality of paths through a network, the first andsecond storage systems being arranged in an active-active configurationrelative to one another; to identify one or more logical storage devicesthat are each accessible via at least first and second different ones ofthe paths to respective ones of the first and second storage systems;and to modify path selection for input-output operations directed to theone or more identified logical storage devices relative to pathselection for input-output operations directed to one or more otherlogical storage devices.
 19. The computer program product of claim 18wherein identifying one or more logical storage devices that are eachaccessible via at least first and second different ones of the paths torespective ones of the first and second storage systems comprises:sending commands on respective ones of the paths over which a givenlogical storage device is accessible to the host device; obtaining fromat least one of the first and second storage systems informationregarding the given logical storage device responsive to the commands;and determining whether or not the given logical storage device isaccessible via at least first and second different ones of the paths torespective ones of the first and second storage systems based at leastin part on the obtained information.
 20. The computer program product ofclaim 18 wherein modifying path selection for input-output operationsdirected to the one or more identified logical storage devices relativeto path selection for input-output operations directed to one or moreother logical storage devices comprises: separating a given one of theone or more identified logical storage devices into at least first andsecond portions; sending input-output operations directed to the firstportion over one or more selected paths to the first storage system; andsending input-output operations directed to the second portion over oneor more selected paths to the second storage system.